Injectors of the type to which the present invention is directed (such injections sometimes being referred to as "pressure infusion apparatus") are used for infusing a liquid medication into a patient by means of a syringe in a uniform manner over an extended period of time, e.g. 24 hours. This requires the discharge of minute volumes of liquid, e.g., 0.06 to 6 ml/h, at a substantially constant infusion rate under conditions of the strictest sterility. The velocity deviation is generally limited to about 1%. The use of highly active drugs requires not only a very constant dosage rate but also an alarm signal to alert attending medical personnel in the event the infusion ends or is interrupted.
In medical injectors, the path travelled by the second holder of the injector which moves the syringe piston is limited by the length of the syringe. Accordingly, it is known practice to equip the injector with a limit switch which, when the syringe has been squeezed empty, turns off the motor that moves the injector's slide. Such automatic switching off of the injector is thus dependent on the length of path travelled by the injector's slide. Because of the manufacturing tolerances of the injector and of the syringe, a safety reserve of the drug must be maintained in the syringe so that the switching off takes place prior to the syringe being completely emptied. Therefore, a residual amount of the drug remains in the syringe. This is a particularly great disadvantage in the case of expensive drugs.
Another problem that occurs with the use of injectors is that the tube connecting the syringe to a patient may become blocked for some reason. For example, the tube may be accidentally pinched. In this situation, the syringe works against a high flow resistance which may result in the syringe piston becoming deformed. This is particularly a problem with single-use syringes which are made of plastic. As a result, the patient does not receive the needed quantity of the drug infusion even though the injector appears to be operating properly. Finally, this condition may lead to overload of the injector. For this reason, overload safety devices have been developed. In the event of an inadmissably high pressure build-up in the syringe, these overload safety devices signal an alarm or cause the injector's motor to switch off.